Method and apparatus for electrostatic coating utilizing projection of liquid solelyby the electric field



March 11, 1958 A. BLANCHARD 2,826,513

METHOD AND APPARATUS FOR ELECTROSTATIC COATING UTILIZING PROJECTION OF LIQUID SOLELY BY THE ELECTRIC FIELD 4 Sheets-Sheet 1 Filed Dec. 28, 1955 March 11, 1958 BLANCHARD 2,826,513

METHOD AND APPARATUS FOR ELECTROSTATIC COATING UTILIZING PROJECTION OF LIQUID SOLELY BY THE ELECTRIC FIELD 4 Sheets-Sheet 2 Filed Dec. 28, 1955 March 11, 1958 A. BLANCHARD 2,826,513

A METHOD AND APPARATUS FOR ELECTROSTATIC COATING UTILIZING PROJECTION OF LIQUID SOLELY BY THE ELECTRIC FIELD 4 Sheets-Sheet 5 Filed Dec.

March 11, 1958 A. BLANCHARD 2,826,513

METHOD AND APPARATUS FOR ELECTROSTATIC COATING I UTILIZING PROJECTION 0F LIQUID SOLELY BY THE E ECTRIC FIELD Filed Dec. 28; 1955 4 SheetsSheet 4 y. E I

United States Patent METHOD AND APPARATUS FOR ELECTROSTATIC COATING UTILIZING PROJECTION OF LIQUID SOLELY BY THE ELECTRIC FIELD Andr Blanchard, Floirac, France Application December 28, 1955, Serial No. 555,999 Claims priority, application France October 13, 1 95!) 11 Claims. (Cl. 117-93) 2,826,513 Patented Mar. 11, 1958 the surrounding objects, the field being completed through.

the air and returned to the earth.

It should also be understood that, in every embodi ment described, there exists, between the orifice of the nozzle and the electrode in contact with the liquid, 2. liquid body acting as a self adjustable resistor.

By way of non-restricting example, there is also described a machine particularly adapted for coating the inside of vessels such as preserving cans, by means of at suitable insulating liquid, e. g. a lacquer, a varnish, or

a an emulsion, said machine incorporating one or more colloidal suspensions, whereby as a practically sole trans- Y fer means, use is made of a high tension electric field established between two electrodes, one of which is either the surface to be coated itself which is in this case conducting, or a conducting surface which is in contact with the surface to be coated, said surface to be coated being either conducting or not and positioned between the electrodes. The other electrode is in contact with a continuous homogeneous body of liquid material contained within an insulating nozzle the orifice of which is directed toward the surface to be coated. The liquid in the nozzle moves fairly slowly in order to drip out of the orifice when the high tension field is not established.

When the field is established, the liquid spouts and flows according to Poiseuilles law in the form of a stream which sprays at a certain distance from the mouth, the explosion occurring prior to the jet meeting the surface to be coated. The orifice is at such a distance from the electrode in contact with the liquid to be projected that no disruptive discharge occurs between the two electrodes when the field is established.

The field is produced by the high voltage source the potential of which is comprised between 10,000 and 100,000 volts, said field being adjustable to a suitable value in order to comply with the electrodynamic qualities of the liquid and the fiow according to Poiseuilles law.

In a first embodiment of the device, the electrode. which is to be in contact with the liquid body is introduced in the axis of the insulating nozzle out of which the liquid to be projected drips when the high tension field is not established.

In order to simplify the construction, instead of introducing into the nozzle the electrode adapted to be in contact with the liquid, the electrode, according to another embodiment of the invention may be constituted by a metal mounting which supports the insulating nozzle, said mounting being in contact with the liquid.

Finally, when it is desired to spray the liquid over a surface which is not conducting, it has been contemplated, according to a further feature of the invention, to-place this non-conducting surface between the electrodes in front of a conducting surface connected to the electrode which is 'not in contactwith the liquid.

It has been found that, even in the absence of the conducting surface forming an electrode, a certain quantity of the liquid spouting out of the nozzle will deposit upon the "non-conducting surface, as well as upon'all of the above mentioned devices.

This machine, in addition to at least one of the devices.

in question, comprises means for successively bringing in front of the. nozzle or nozzles the vessels to be varnished or coated, a reciprocating carriage adapted to cause the nozzle or nozzles to penetrate into the vessels and to come out therefrom, means for temporarily applying the high tension field, means for determining the supply of liquid to the nozzle or nozzles, and preferably a generalcontrol system for timing all said means.

Further objects and advantages of the present invention will appear from the following description of some embodiments selected by way of examples with reference to the accompanying drawings in which: i

Fig. 1 is a diagram illustrating a first embodiment of the device.

Fig. 1A shows a modification of this device. Fig. 2 diagrammatically shows a second embodiment. Fig. 2A shows a modification of the device of Fig. 2.

Fig. 3 is a diagram of the machine adapted for coating or varnishing the inner surface of vessels, and

Fig. 4 shows, with more details, the machine as a whole and, in therein. i

Referring first to Fig. 1 it will be seen that the device comprises a nozzle 1 made of an insulating material such as glass, synthetic resins and notably the transparent synthetic resins (for instance that available under the trade name of Plexiglas), quartz, silica, porcelain, etc.

laminary flow rate which corresponds to a dripping rate When the electrical field is not established. By way of example, for a liquid with a viscosity near 20 seconds (Ford cup No. 4) and an orifice comprised between 0.12 and 0.15 mm. in diameter, there will flow out of.

the orifice a volume of liquid of 0.20 to 0.40 cmfiper second approximately, which corresponds to one or two. drops per second, the head being at a pressure of 10,

to 15 g./cm. approximately. The existence of small cylindrical tube 13 has is given the flow which corresponds to the quantity of liquid necessary in the time unit in order to obtain a layer of determined thickness over a determined area, there is also given the head which corresponds to the height of the constant level reservoir 3, there is also given the length which remains constant and the diameter of the tube is thereby determined, which constitutes the parameter of the hydrodynamic adjustment.

Penetrating into the insulated nozzle, axially thereof, is

particular, the carriage incorporated the advantage of permitting readily applying the Poiseuilles formula which, as it is known, gives the flow per unit of time in terms of the viscosity of the liquid, the length of the tube, its diameter and the head. In thiscase, there i an electrode 4 connected to one of the poles of a source of adjustable high tension, either direct or alternating, said source being diagrammatically represented by the box 5. Positioned in front of the, orifice of thenozzle is the conducting surface 8 which is, grounded as at 9. The pole of the source. which is not connected to the electrode 4.is also grounded.

The operation of the device is as follows When the field is established, the liquid, which previously was dripping out of the orifice 13, now spouts, under the action of the field, out of said orifice while forming first a cylindrical stream 10 which. at a certain distance from the orifice, sprays into a cone 12.

The jet being subjected to gravity is slightly bent toward the earth.

Without the following explanation being liable to restrict the scope of the invention, it seems that the phenomena which occur may be explained in a following manner: The electric energy generatedby the field would act in a manner somewhat similar to thermal energy which, upon reaching a predetermined value, causes a liquid suddenly to vaporise. It appears furthermore, that, for a determined body and a given field, the length 10 is constant, it being assumed that the conditions of viscosity and pressure at the exit from the nozzle, and the diameter of the orifice remain unchanged.

If the tension of the field is increased, the length 10 first decreases until the tension reaches a maximum above which said length remains unchanged.

There exists, therefore, for each liquid, a critical tension for which the length 10 is minimum.

The two extreme aspects would be: either a jet without explosion, or a jet diverging from the exit 13 from the nozzle 1.

The length of the body of liquid comprised between the end of the electrode 4 and the orifice of the nozzle 1 is determined experimentally in such a manner that no disruptive discharge occurs between the electrode 4 and a receiving conducting surface 8, when the field is established. This body of liquid has a function of self regulation in that, upon an excessive quantity of liquid being discharged out of the nozzle for instance owing to a variation of the potential, such liquid would be immediately replaced by a greater flow of liquid out of the reservoir 3. The body of liquid, therefore serves as a self regulator by means of variations of resistance.

A device of this kind has the advantage of lending it self, without any danger, to industrial purposes in spite of the high potential employed. The current consumption is very low, for instance of the order of 50 microarnperes per orifice, and depends only on the fiow rate of the liquid which is proportional to the areas of the surfaces to be coated.

The device of Fig. 1A is similar to that of Fig. 1, with the difference that one of the poles of the high tension source 5 is connected to the conducting surface 8 whereas the electrode 4 is grounded.

Fig. 2 shows another embodiment of the device according to the invention. Like members are identified by like reference members. In this device, it is the conducting surface 8 that is connected to the high potential of the source 5 through a brush 14. One of the advantages consists in that any risk of dripping from the orifice of the nozzle is avoided, such drops are likely to initiate discharges if they are under a high potential. In the device ofFig. 2, wherein it may be considered that the surface to be treated exerts an attraction upon the varnish, no drop maypossibly form at the exit from the nozzle.

Furthermore, in Fig. 2, instead of representing an electrode 4 penetrating inside the varnish, this electrode is constituted by a metal mounting 15 in contact with the liquid. This mounting holds the insulating nozzle 1 and is grounded as at 16. This construction is much more practical. Itv goes without. saying that, in this: case, the reforming the surface to be coated, it will be possible to 1 varnish or coat the inside of this vessel 17.

It may be assumed that the electrode in contact with the liquid and the surface to bevarnished constitutes plates of a capacitor. The disposition of Fig.2 corresponds merely to a superposition of the dielectrics inside the ca- 'pacitor.

The only ditference' between Fig. 2 and Fig. 2A consists in that, in Fig. 2A, it is the electrode '15 that is connected to one pole of the source whereas the electrode 8 is grounded.

In the machinediagrammatically illustrated in Fig. 3,

the insulating nozzle 1 is mounted in the metal cock 18, 1

which serves as an electrode and is grounded by a reciprocable carriage 19 and its supporting rolls 21. On the carriage is also mounted thereservoir 3 which is a constant level vessel supplied with liquid (lacquer) from the reservoir 22.. A cam 23 makes it possible to tilt the nozzle ,1 to the desired extent. Actually, as a rule, instead of a single nozzle 1, there are provided a plurality of parallel nozzles fed by a multiple cock 18. Opening and closing ofthe, cock 18 are effected by a cam 24. A cam 25 actuates a switch 36 cut off and establishes the high potential at the suitable. moments. By way of example, the high potential source may be derived from a direct high tension system such as that diagrammatically shown as at 5 on Fig. 3.

Finally, the vessel 8. to be varnished internally is supportedv by insulating rollers 26 rotatably driven.

For industrial purposes, it. is advantageous to adjust the parameters conveniently; so. as to obtain a jet 1t terminated by a diverging cone 1?- best .suiting the. particular article and, thus, secure a varnishing operation yielding a good surface finish which is homogeneous and continuous.

Fig. 4 shows a machine asv actually constructed: the rollers .21 of the carriage 19 roll. on the rails 27 bolted to the frame 30. The movement of the carriage which carries the set of nozzles 1 is effected by the outer cam 31 which forms the edge of the plate 32 rigid with the shaft 29. The nozzle 1 may be tilted by means of a horizontal bolt 34 and a lever terminated by a roller 35 in with the cam 31, effects through the medium of a switch 36 and relay 36 the application of the high tension onto the surface 8 through a slider 42. This surface remains alive during a predetermined time in response to the movement of the cam 25..

At the same time, at a given moment of the rotational cycle of the cam 31, the clock 40 which supplied the nozzles is opened by the cam 24 rigid in rotation with the shaft 29, through the medium of a cam-operated switch 37, relay 37', and solenoid 38 connected tothe cock, 40 by a lever 39.

Thus, the varnish or emulsion is introduced into the set of nozzles, at the required moment in order to be subjected to the field, at a suitable moment of the rotational cycle so as to provide strictly the projection desired.

Considering Fig. 4, the sequence of the operations is as follows:

The vessel 8 which rotates on its own axis on the insulating rollers 43 is brought facing, the nozzles 11 and 1. Apparatus for applying to a surface, a liquid having a dielectric constant between 1 and 5 and a resistivity above 1.44 ohms, comprising at least one nozzle of insulating material having at the fore end thereof a nozzle orifice directed toward said surface, means for supplying said nozzle with a homogeneous and continuous body of said liquid at a pressure below that required to produce the flow of a continuous stream of liquid from said orifice, a first electrode associated with said nozzle and in contact with the liquid in said nozzle and a second electrode associated with said surface, means for applying a voltage of the order of 10,000 to 100,000 volts between said electrodes, said orifice being disposed centrally in the electric field between said electrodes and said first electrode being spaced rearwardly of said orifice in a direction away from said surface a distance greater than the arcing distance of the voltage applied between said electrodes, said electric field being of a magnitude to force a jet of liquid out of said orifice toward said surface and disperse said jet into a spray before the liquid reaches said surface, said liquid being transported from said orifice to said surface substantially solely by said electric field.

2. A method of transporting onto a surface a viscous liquid having a dielectric constant of the order of 1 to 5 and a resistivity not less than 1.44 10 ohms, consisting in introducing a homogeneous and continuous body of said liquid into an insulating enclosure having a discharge orifice directed toward said surface, applying to the liquid in said enclosure a pressure sufficient to cause said liquid to flow to said orifice but insuflicient to produce the flow of a continuous stream of liquid from said orifice, establishing an electric field having a potential of 10,000 to 100,000 volts between a first electrode which is associated with said enclosure and in contact with said body of liquid and a second electrode which is associated with said surface, said first electrode being spaced rearwardly of said orifice in a direction away from said surface a distance greater than the arcing distance of the voltage applied between said electrodes, said electric field being of a magnitude to force a jet of liquid out of said orifice toward said surface and dispersing said jet into a spray before the liquid reaches said surface, said liquid being transported from said orifice to said surface substantially solely by said electric field.

3. A device such as claimed in claim 1, wherein the first electrode comprises an elongated conductor positioned on the axis of the nozzle, with its fore end positioned at such a distance of the orifice of the nozzle that the thickness of liquid comprised between said fore end and the orifice prevents any disruptive discharge from taking place between the electrodes when the field is established.

4. A device such as claimed in claim 1, wherein the second electrode is connected to one pole of a high tension source the other pole of which is grounded and the first electrode also is grounded.

5. A device such as claimed in claim 1, wherein the first electrode is connected to one pole of a high tension source the other pole of which is grounded and the second electrode also is grounded.

6. A device such as claimed in claim 1., wherein the first electrode is constituted by a metal mounting positioned at the end of the nozzle opposite the orifice thereof and at such a distance from said orifice: that no electrical discharge can take place between the electrode when the field is established.

7. A device such as claimed in claim 1, in which the second electrode is constituted by the surface to be coated which itself is a conducting surface.

8. A device such as claimed in claim 1, wherein the second electrode is constituted by a conducting surface whereas a non-conducting surface is positioned on the side of the conducting surface facing the orifice of the nozzle.

9. Machine for coating the inner surface of vessels by means of a liquid the dielectric constant of which is comprised between 1 and 5 with a resistivity above l.44 10 ohms by using as a practically sole transport means, an electrical field produced by a high tension source the potential of which is comprised between 10,000 and 100,000 volts between two electrodes, comprising in combination at least one nozzle made of a material which does not conduct electricity, having at the fore end there of an orifice directed toward the inner surface of at least one of said vessels, supply means for supplying said nozzle with a homogeneous body of said liquid, a cock serving as an electrode secured to the nozzle at a certain distance from the orifice of said nozzle and making and interrupting the communication of said nozzle with the supply means, a further electrode connected to a conducting vessel having the shape of one of the vessels the inside of which is to be coated and receiving said vessel, a carriage carrying said nozzle, the cock and the supply means, means for reciprocating the carriage with respect to the vessel, means operative in timed relation with the preceding means for opening and closing the cock, and a source of high tension the potential of which is comprised between 10,000 and 100,000 volts for establishing the field between the electrodes.

10. Machine such as claimed in claim 9, furthermore comprising a switch and means operative in timed relation with the movements of the carriage and the operation of the cock for connecting or disconnecting the high tension source.

11. Machine such as claimed in claim 9, further comprising means cooperating with the nozzle in order to vary the inclinations of said nozzle in timed relation with the movements of the carriage.

References Cited in the file of this patent UNITED STATES PATENTS 1,416,929 

2. A METHOD OF TRANSPORTING ONTO A SURFACE A VISCOUS LIQUID HAVING A DIELECTRIC CONSTANT OF THE ORDER OF 1 TO 5 AND A RESISTIVITY NOT LESS THAN 1.44X106 OHMS, CONSISTING IN INTRODUCING A HOMOGENEOUS AND CONTINUOUS BODY OF SAID LIQUID INTO A INSULATING ENCLOSURE HAVING A DISCHARGE ORIFICE DIRECTED TOWARD SAID SURFACE, APPLYING TO THE LIQUID IN SAID ENCLOSURE A PRESSURE SUFFICIENT TO CAUSE SAID LIQUID TO FLOW TO SAID ORIFICE BUT INSUFFICIENT TO PRODUCE THE FLOW OF A CONTINUOUS STREAM OF LIQUID FROM SAID ORIFICE, ESTABLISHED AN ELECTTIC FIELD HAVING A POTENTIAL OF 100,000 TO 100,000 VOLTS BETWEEN A FIRST ELECTRODE WHICH IS ASSOCIATED WITH SAID ENCLOSURE AND IN CONTACT WITH SAID BODY 